Bubbles in wine were known to vintners long before they could reliably capture and preserve this phenomenon in the bottle. As a natural byproduct of the fermentation process, carbon dioxide is released in the liquid to provide a "sparkle." In the Northern climates, cold weather sometimes arrives early after harvest, stopping fermentation before the sugar is completely used up. Warm weather in the spring often causes it to start up again, resulting in carbonated wine. The English imported wine in casks. They found also that adding sugar to tart, acidic wine would often soon cause it to sparkle and they developed a liking for it. English bottles were much stronger than those in France and not as inclined to burst when the pressure built up.